Turbo-machine rotor assembly



July 15, 1958 T. N. HULL, JR 2,843,356

. TURBO-MACHINE ROTOR ASSEMBLY Filed April 5, 1954 21W i 1,, WV 9 I m. /3

lhvehtor Thomas N. Hull Jr.

United States Patent M TURBO-MACHINE ROTOR ASSEMBLY Thomas N. Hull, Jr., Schenectady, N. Y., assignor to General Electric Company, a corporation of New York Application April 5, 1954, Serial No. 420,835

6 Claims. (Cl. 253-77) This invention relates to a high temperature turbomachine rotor assembly, particularly to such a rotor with dovetail type bucket fastenings used in a turbine which is subject to very high temperatures and frequent changes in load.

In practice, steam or gas turbines are subject to variable station loads which automatically regulate the power fluid supply and thereby vary the heat to which the turbines are subjected. The frequent changes in load result in many additional stresses being placed on the rotor. In addition to the usual centrifugal and tangential stresses, the bucket wheels are subject to stresses in consequence of becoming heated by the power. fluid and of cooling. When, for example, the power fluid is shut ofi, the temperature of the buckets and of the rim portion of the bucket wheel becomes lower than the temperature of the wheel nearer the hub so that the rim portion contracts more rapidly than the more expanded central portion of the wheel, thus causing what is known in the art as hoop stresses, causing repeated stressing of the outward portion of the wheel around its rim and then occasioning peripheral cracking of the Wheel. Also, when the turbine is started the center of the rotor is cold and the rim heats causing high compressive stresses, which when repeated may cause thermal fatigue failures.

These heating and cooling cycles encountered in normal operation cause frequent alternate expansions, contractions, and fatiguing the metal of the rotors so that they frequently crack, occasioning delays, renewals, and very considerable expense.

It is an object of the present invention to improve the life expectancy of a high temperature turbine wheel by preventing or minimizing the thermal stresses set up between the rim and the central portion of the wheel.

It is also an object of this invention to provide a turbine rotor with improved bucket fastening means for preventing axial movement of the buckets relative to the rotor.

A further object is to provide a slotted rim high temperature bucket wheel with means for avoiding windage losses therethrough.

jA still further object is to providea turbo-machine rotor with a balanoing means which can be easily and efficiently installed and removed;

Other objects and advantages will become apparent from the specification and claims and from the accompanying drawings which illustrate an embodiment of the invention, in which Fig. l is a perspective viewshowing the turbine wheel containing the bucket and filler member and the bucket and filler member'before insertion intothe turbine wheel;

Fig. 2 is a front view partially in section showing the last bucket installed on the turbine wheel;

Fig. 3 is a partial sectional view taken along line 3-3 of Fig. 2 illustrating the side view of the last bucket installed on the turbine wheel;

Fig. 4 illustrates the preliminaryand final position" of 2,843,356 Patented July 15, 1958 the bucket during installation. of the last bucket on the turbine wheel; and

Fig. 5 is a sectional view showing the contour of the turbine wheel and conventional cooling means therefor.

Generally stated, the invention is practiced by providing a deep radial slot in the bucket wheel at the bottom of each dovetail groove to-allow therim to expand circumferentially, with special filler pieces to prevent substantialleakage of motive fluid through the slots.

Referring first to Fig. 1, a turbine wheel 1 is provided with an evenly spaced circumferential row of axially extending V-shaped dovetail grooves 2 (which areby way of example only), having serrations 3 extending along the sides of the V. Located in each groove is a bucket having a. dovetail base. portion of substantially the same configuration as the dovetail grooves; At the base of the V-shaped grooves there are cut comparative- 1y narrow and deep radial slots 4 which extend axially through the rim portion 1a of the bucket. wheel and radially across the high temperature rim portion 1a of wheel 1. These slots allow thermal expansion to take place at the rim of the turbine wheel without setting up large thermal stresses. Such thermal stresses at the rim of the wheel, if not prevented, would have in time caused cracks to form at the bottom of the dovetail grooves which would have made eventual replacement of the wheel necessary.

In order to prevent a substantial amount of power fluid fromleaking through the radial slot 4 and thereby cause a drastic loss in efiiciency of operation of the turbine, the filler members 5 are inserted in the slots.

The filler member-5 is the same depth as the slot 4 but is very slightly narrower than the slot 4, to form the clearance space shown at 24 in Fig. 2, so as to allow for free thermal expansion of the rim portion 1a without creating excessive thermal stresses.

The bucket 6 is keyed to filler member .5 by means. of a groove 13in bucket 6 and tongue 14 on filler member 5.

The key means for connecting the bucket and filler plate assembly to the turbine wheel consists of cooperating recesses 7 and 8 in the turbine bucket and wheel respectively,. and keys 9 for insertion. into said. recesses. Recesses 7 and 8 are substantially equal in length to the width of the land portion 23 of turbine Wheel 1 between V-sliaped grooves 2. a

It will be apparent to those skilled in the art that other equivalent means for locking the bucket and filler plate to the. turbine wheel. may be used. For instance, the filler plate may be locked directly to the turbine wheel by key means or the bucket can be locked to the turbine Wheel by using. grub screws or peening the end portions ofthe. bucket bases.

Thefiller member 5 is shown. ashaving. an internal cut out portion 10, for lightening the filler rnembeiz. This cutout portion 10 can be of various depths as, fonexample, of a depth indicated by the line 11 if additional weight is needed to balance the. turbine wheels More specifically, there may be provided two sets of filler pieces, one set to the shape represented in full lines in Fig; l, and the other. to the modified shape representedv by. the broken line 11. In addition, the filler'pieces may be made of different materials, having differing specific weights Then selective assembly of these various types. of filler pieces can be used tosecure proper dynamic balance. of the rotor.

The filler member 5 is shown as a solid piecebut can beconstructed of two or more pieces of sheet metal spot Welded together, or in any manner known to one skilled in the art. The contour 12 of the tiller .plate. (see Fig; 5) is designed to conform to the shape of the turbine 3. wheel. This is desirable to allow for smooth flow of the cooling gases around the turbine wheel.

All the buckets, withthe exception of the last one, can

be located axially on theturbine wheel by the key means 7, 8, and 9.

' Figs. 2, 3 and 4 illustrates the means whereby the last bucket is secured against axial displacement.

The last bucket to be installed has on the underside of its base the recess 15. When the bucket is installed on the turbine wheel, this recess is in alignment with a 'radially extending circular hole 16 in the turbine wheel 1. At the bottom of radial hole 16 there is an axially extending circular hole 17 leading from hole 16 to the side of the turbine wheel, the diameter of the hole 16 being substantially equal to the width of the recess 15 and the diameter of the hole 17 being substantially equal to the depth of recess 15.

'A turbo-machine rotor with dovetail type bucket fas- 'tenings embodying the invention of deep radial slots at the bottom of each bucket dovetail is assembled in the following manner.

The bucket.6 is placed adjacent the filler member 5 with tongue 14 fitting recess 13 and both are caused to slide axially into the dovetail groove 2 and slot 4, respectively, the class of fit between the bucket and the wheel beingsuch that the bucket is loosely secured to the wheel. This connection prevents radial displacement of the bucket.

The bucket is then secured against axial displacement by inserting key 9 in the cooperating recesses 7, 8. Then the second and successive buckets are similarly keyed to the wheel, with each succeeding bucket abutting against the previously installed key. This method cannot be used for the last bucket since there is no way to insert the key 9. Therefore, the last bucket is installed in the following manner.

Pin 19 is first inserted radially into the hole 16, the pins dimensions being substantially the same as the hole 16 and beveled'at the bottom end, as shown at 19a in Fig. 4. The bucket is then slid into its dovetail groove,

with recess 15 in alignment with hole 16. Pin 18, of

substantially the same dimensions as hole 17, is then in- ;serted into hole 17 so it engages the tapered portion 19a of pin 19. Driving the pin 18 into hole 17 thus forces pin 19 radially outwardly into the recess 15, holding the bucket in place.

The pin 18 may be held in place by any suitable method, as by peening an adjacent portion 'of the turbine wheel over the end of the pin, as shown at 20 in Fig. 3.

Referring now to Fig. 5, there is illustrated a sectional view of the upper half of a' conventional turbine wheel embodying the invention. The turbine wheel is cooled in the conventional manner by the flow of cooling gases around the spill bands 22. The flow of cooling 'gases is directed in an appropriate path into contact with the central portion of the wheel, as indicated diagrammatically by the flow path arrows 22a and 2212. I The edges of each filler member 5 define a substantially smooth continuation of the adjacent side surfaces of the turbine wheel so that when inserted into the radial slot By way of example only, the ratio between the overall radial depth of the slot, identified d in Fig. 1, and the racllius g of the wheel (see Fig.5) may be on the order of Thus, it will be seen that the invention prevents high thermal stresses 1n the uncooled rim portion of a turbo- 4' machine rotor assembly with dovetail type bucket fastenings by the provision of deep radial slots at the bottom of each dovetail groove to allow the rim to expand circumferentially, relative to the cooled central portion of the wheel, while leakage of power fluid through the radial slots is substantially prevented by filler members loosely secured in each of the slots.

A turbine rotor provided with the inventions described herein has been in operation over 5,000 hours without thermal fatigue cracks developing at the base of the dovetail. This is roughly twice the length of service which in the past has produced cracks in rotors not incorporating the invention.

While a single embodiment of the invention has been described in detail herein, it will be obvious to those skilled in the art that many changes and substitutions of equivalents might be made; and it is desired to cover by the appended claims all such modifications as fall within the true spirit and scope of the invention.

What I claim as new and desire to secure by Letters Patent of the United States is:

1. In a turbo-machine rotor assembly, a wheel member having a rim portion defining a circumferential row 'of axially extending dovetail grooves, said rim portion also defining a slot extending axially through the rim at the bottom of each dovetail groove, said slot having a depth at least substantially that of the depth of the dovetail grooves and extending radially across the high temperature rim portion of the wheel, a plurality of buckets each having a dovetail base portion of substantially the same configuration as said dovetail grooves and disposed in one of said grooves, a filler member loosely disposed in. and substantially filling each of said slots to substantially prevent leakage of power fluid therethrough, and means preventing axial displacement of the filler member relative to the wheel.

2. A rotor assembly in accordance with claim 1 in which the means for preventing axial displacement of the filler member relative to the wheel comprises cooperating key means disposed between the bucket and filler member.

3. A rotor assembly in accordance with claim 1 in which at least two of the filler members are of different weights whereby the wheel may be dynamically balanced by selective assembly of the filler members.

4. A rotor assembly in accordance with claim 1 in which the exterior configuration of the filler member conforms to the exterior surface of the turbine wheel at the location of the slot, whereby the edges of the assembled filler pieces define a substantially smooth continuation of the adjacent side surfaces of the wheel to allow for smooth flow of gases around said wheel.

5, In a turbo-machine rotor assembly, a wheel member having a rim portion defining a circumferential row of axially extending dovetail grooves, said rim portion .also defining a slot extending axially through the rim at the bottom of each dovetail groove, said slot extending radially across the high temperature rim portion of the wheel and having an over-all radial depth .on the order of /6 the radius of the wheel, a plurality of buckets each having a dovetail base portion of substantially the same configuration as said dovetail grooves and disposed in one of said grooves, a filler member loosely disposed in and substantially filling each of said slots to substantially prevent leakage of power fluid therethrough, said filler member having an exterior configuration which conforms to theexterior surface of the wheel 6. In a turbomachine rotor assembly, a Wheel memher having a rim portion defining a circumferential row of axially extending dovetail grooves, said n'm portion also defining a slot extending axially through the rim at the bottom of each dovetail groove, said slot extending radially across the high temperature rim portion of the wheel, a plurality of buckets each having a dovetail base portion of substantially the same configu ration as said dovetail grooves and disposed in one of said grooves, a filler member loosely disposed in and substantially filling each of said slots to substantially prevent leakage of power fluid therethrough, each filler member having a central cutout portion, said cutout portions being of various depths whereby when a change in weight is needed for dynamic balance of the rotor at the place on the wheel where the filler member is inserted, the cutout portion can be of such a depth that the change in weight of the filler member will compensate for any dynamic unbalance of the rotor, and means preventing axial displacement of the filler member relative to the wheel.

References Cited in the file of this patent UNITED STATES PATENTS 1,619,133 Kasley Mar. 1, 1927 2,686,655 Schorner Aug. 17, 1954 2,753,149 Kurti July 3, 1956 FOREIGN PATENTS 166,859 Great Britain July 28, 1921 241,993 Switzerland Sept. 2, 1946 609,446 Great Britain Sept. 30, 1948 643,914 Great Britain Sept. 17, 1950 718,939 Germany Mar. 25, 1942 834,408 Germany Mar. 20, 1952 986,022 France Mar. 21, 1951 

